The Australian Border Force was created on 1 July
2015, combining the functions of the Department of Immigration and Border
Protection and the Australian Customs and Border Protection Service. As a
result, the Australian customs flag has been changed to reflect the name Border
Force. The Customs Regulations were amended to change the word "CUSTOMS" to
"AUSTRALIAN BORDER FORCE", but a flag flying
at Sydney Airport has the text simply as "BORDER FORCE".(see
https://www.comlaw.gov.au/Details/F2015L00854 Customs and Other Legislation
Amendment (Australian Border Force) Regulation 20, Schedule 1, Section 10,
modifying Section 11 of the Customs Regulations.)

Clearly a defaced
national flag doesn't need the word "AUSTRALIAN", so the flag in use at the
airport is more sensible than the de jure customs flag. There would be
seriously legal problems for ships using anything other than the de jure
flag, though, so perhaps the regulations will be/are being changed again.Jonathan Dixon, 17 July 2015

In early March I noticed that the BORDER FORCE flag at Sydney Airport Customs
House had been replaced with one that included AUSTRALIAN. However as there was
little breeze I could not make out the appearance of the letters.Jeff
Thomson, 22 March 2016

Apparently this interim BORDER FORCE version of the flag came about because such new flags had to be ordered well in advance, before it was clear what exactly the amended flag prescription would prescribe. When the Customs Regulation 2015 amendments took effect from 1 July 2015 the altered flag prescription stated;

11. Flag.
For section 14 of the Act, the flag is the Australian National Flag with the words "AUSTRALIAN BORDER FORCE" in bold, white letters between the Commonwealth Star and the lower part of the Southern Cross.

Unlike with the previous Customs flag, nowhere in the legislation is the flag referred to as the Australian Border Force flag or any other name. It is only referred to as a reference to section 11 as shown above.Jeff Thomson, 27 May 2017